The present invention relates to a pneumatic tire improved in the rigidity on the bead toe portion side of a bead core, particularly to a heavy-load radial tubeless tire for use in trucks, buses, etc.
A heavy-load radial tubeless tire has a bead portion structure, e.g., as shown in FIG. 4(A). In FIG. 4(A), a carcass layer 2 is turned up around a bead core 1 in a bead portion 10 from the inside of the tire towards the outside of the tire, and a reinforcing layer 3 is provided on the outside of the carcass layer 2. A lower bead filler 4 and an upper bead filler 5 are each provided on the upside (tread side) of the bead core 1, and a reinforcing layer 6 is provided on the outside of a turn-up edge 2u, the reinforcing layer 3, and the upper filler 5. Numeral 7 designates a bead toe portion, numeral 8 a bead heel portion, and numeral 9 a bead base portion. The bead base portion 9 is inclined at substantially the same angle as that of a rim base portion (not shown) relative to the axial direction E of the tire for the purpose of enhancing the compatibility with a rim.
FIG. 4(B) and FIG. 4(C) are each an example of a bead core. In FIG. 4(B), the bead core 1 has a hexagonal cross section comprising a plurality of bead wires 11 each having a circular cross section, while in FIG. 4(C), the bead core 1 has a rectangular cross section comprising a plurality of bead wires 11 each having a rectangular cross section. The bead cores 1 respectively shown in FIG. 4(B) and FIG. 4(C) are each inclined at substantially the same angle .alpha. as that of the rim base portion relative to the axial direction E of the tire.
It is well known that in the above-described tires, a certain kind of tensile force acts on the bead core when the tire is filled with an internal pressure or the tire is rolled under load. In particular, in the case of a radial tire wherein substantially inextensible cords are used as carcass cords for the carcass layer, the carcass cords are provided in the direction normal to the circular bead core. Therefore, as shown by an arrow in FIG. 5(A), the force acting through an internal pressure when the tire is filled with an internal pressure or the tire is rolled under load is conveyed to the bead core 1, so that the force acting in the direction normal to the bead core is larger than that in the case of a bias tire comprising a ply composed of bias laminated carcass cords. In FIG. 5(A), numeral 12 designates a rim and numeral 13 a rim base portion.
In particular, in radial tubeless tires for trucks and buses which are in rapidly increasing demand resulting from an improvement in the express highway in recent years, the internal diameter is smaller than the rim diameter of the rim base portion 13 having an inclination of 15.degree. relative to the axial direction E of the tire for the purpose of hermetic sealing, and the bead base portion 9 is inclined at substantially the same angle as that of the rim base portion 13. For this reason, the bead base portion 9 is compressed from a position shown by a dotted line to a position shown by a solid line. Consequently, as shown in FIG. 5(B), this not only increases the reaction force from the rim 12 remarkably but also causes a large force m to act outward in the radial direction F of the tire.
As a result, during rolling of the tire, the rubber of the bead toe portion 7 is repeatedly deformed under large compression against the rim base portion 13 accompanying the flexing deformation of the bead core 1, which unfavorably brings about a combination of deterioration of the physical properties of the rubber due to the rise of the temperature of the air present within the tire with, as shown by a solid line in FIG. 5(C), an unrecoverable permanent deformation of the rubber of the bead toe portion 7 during a long-term use of the tire. In FIG. 5(C), a dotted line represents the shape of the rubber before running, and a solid line represents the shape of the rubber after running. Further, letter T represents the length of the permanently deformed portion in the axial direction E of the tire, and letter S represents a permanent deformation length in the radial direction of the tire. This kind of deformation occurs in a very narrow range at the top of the bead toe portion 7 in an early stage of the use. However, once the deformation occurs, the area of contact between the bead base portion 9 and the rim base portion 13 is decreased, which brings about an increase in the reaction force of the rim per unit width of the bead base portion 9, so that the deformation further progresses towards the bead heel portion 8.
Thus, the occurrence of the deformation decreases the area of the bead base portion 9 which can be substantially brought into contact with the rim base portion 13. This causes the hermetic sealing of the tire to be spoiled, the injection of air during rim assembling to become difficult, and further vibration to occur due to unstable fit of the bead base portion 9 into the rim base portion 13.
Further, a decrease in the area of contact 25 between the rim base portion 13 and the bead base portion 9 spoils firm fixation of carcass cords under the bead core. This increases the movement of the carcass cords and brings about an increase in the movement of the turn-up edge of the carcass layer, which 5 induces the separation of the edge portion.
In the prior art, in order to eliminate the above-described problems, for example, as shown in FIG. 6, bead wires 11 have been further laminated at the dotted line portion of the bead core 1 to reinforce the bead core 1 without destroying the hexagonal cross section of the bead core 1. However, in this case, the amount of the overhang towards the bead toe side is increased from W.sub.1 to W.sub.2, which brings about new problems such as an increase in the weight of the bead core 1. In addition, the force acting at a head part of the bead toe portion side of the bead core as shown in FIG. 5(B) is not decreased.
Under the above circumstances, the present inventors have analyzed a number of tires with respect to the deformation of the bead toe portion of the radial tubeless tire and, as a result, have found that in a radial tubeless tire after a long-term use, the initial shape (in a new tire) of the bead core is not retained, that the bead core 1 is deformed as shown by a dotted line in FIG. 7, that the bead toe portion 7 is raised and the inclination of the bead base portion 9 is slightly decreased to approach a horizontal line. Further, the present inventors have made detailed studies on the physical properties of the bead wires of each portion of the bead core of a radial tubeless tire after a long-term used and, as a result, have discovered that as shown in FIG. 8, a lowering in the physical properties of the bead wires on the bead toe portion side is extremely larger than that on the bead core center region and the bead heel portion side (i.e., the index of elongation at breakage of the bead wires on the bead toe side is small and an elastic characteristic thereof is lowered). Further, it has been found that the lowering in the physical properties of the bead wires gradually spreads from the head part region of bead toe portion side towards the bead core center region with an increase in the service period (travel distance) of the tire. For this reason, the present inventors have reached a conclusion that the deformation of the bead toe portion of a radial tubeless tire can be effectively prevented by enhancing the strength and the rigidity of the bead core on the bead toe portion side through an increase in the number of the bead wires provided at the bead core portion on the bead toe portion side.